In automotive vehicles, it is known to provide the electrical system with a rechargeable battery, such as a lead-acid accumulator, which is recharged by an engine-driven battery charger, such as an alternator or dynamo. The battery is used to start or "crank" the engine, such as an internal combustion engine, and to supply electrical power to the electrical system when the engine-driven charger cannot supply sufficient power, for instance, at relatively low engine speeds. For higher engine speeds, the charger supplies power to the electrical system and recharges the battery until it is recharged to its normal maximum working capacity
It is also known to provide a rechargeable auxiliary battery, such as a lead-acid accumulator, to assist in starting the engine. The auxiliary battery is also recharged by the engine-driven battery charger and, once recharged to its maximum normal capacity, is isolated from the electrical system.
According to the invention, there is provided a vehicle battery controller for controlling the connection of a rechargeable auxiliary battery to a rechargeable main battery of a vehicle electrical system including an engine-driven battery charger, the controller comprising a first comparison means for connecting the auxiliary battery to the main battery during engine starting if the auxiliary battery voltage is greater than a first threshold, and a second comparison means for disconnecting the auxiliary battery from the main battery during engine running if a battery voltage is less than a second threshold.
The battery voltage which is measured during engine running when the auxiliary battery is connected to the main battery is effectively a common battery voltage and may be measured at any suitable point of the electrical system, such as at the auxiliary battery or at the main battery. The auxiliary battery voltage is preferably measured at the auxiliary battery so as to avoid the effects of voltage differences occurring between the auxiliary battery and other points in the electrical system, for instance caused by the flow of current through the finite resistance of conductors.
Preferably the second comparison means is arranged, after disconnection of the auxiliary battery from the main battery, to connect the auxiliary battery to the main battery if the main battery voltage is greater than a third threshold. Preferably the third threshold is substantially equal to the second threshold.
Preferably the controller is arranged, after disconnection of the auxiliary battery from the main battery, to measure a first voltage change of the auxiliary battery voltage and a second voltage change of the main battery voltage. Preferably the controller is arranged to connect the auxiliary battery to the main battery if the first voltage change is negative and the second voltage change is positive. Preferably the controller is arranged to decrement the second threshold if the first voltage change is negative and the second voltage change is positive. Preferably the controller is arranged to increment the second threshold if the first voltage change is positive and the second voltage change is negative.
According to another aspect of the invention, there is provided a vehicle electrical system including a controller according to the invention.
Preferably the controller is arranged to control a switch, such as a contactor, connected between the auxiliary battery and the main battery. This is thus possible to provide an arrangement which will reliably allow a vehicle engine to be started after lack of use for a substantial time period. During such lack of use, various loads such as clocks and alarms systems remain connected to the main battery and may, after a sufficient time period, discharge the main battery to such an extent that it is unable to start the engine. Such a situation is aggravated in cold conditions where the main battery capacity is further reduced. During lack of use, the auxiliary battery is isolated and so is not discharged by any loads within the vehicle. In normal circumstances and in cold conditions, the auxiliary battery retains sufficient charge to allow the vehicle engine to be started, irrespective of the condition of the main battery.
In accordance with another object of the invention there is provided a method of controlling the use of an auxiliary battery in a vehicle electrical system which includes a main battery, an auxiliary battery, an engine starting apparatus and an engine-driven battery charger, the method comprising the steps of:
(a) comparing the auxiliary battery voltage with a first threshold before the engine is started and preventing connection of the auxiliary battery to the main battery if the auxiliary battery voltage is less then said threshold;
(b) comparing the excess of the main voltage battery over the auxiliary voltage battery with a predetermined margin;
(c) connecting the auxiliary battery to the main battery after an engine starting operation if the excess exceeds the margin, but before the engine starting operation otherwise;
(d) comparing the common battery voltage when the engine is running and the batteries are interconnected with a second threshold; and
(e) disconnecting the auxiliary battery from the main battery if the common battery voltage is less then said second threshold.